Mishnah 8 in the tenth chapter of Maseches Nedarim deals with the annulment of vows: this annulment may be done throughout the entire day. However, it must be done on that very day. If the vow was not annulled that day, it is too late, this is considered as if he upheld the vow, the vow stands, and there is no longer any possibility of annulling it.
What is "the entire day"?
This definition has a lenient side and a stringent side: sometimes it is lenient and grants the husband a long, generous span of time, and sometimes it is extremely stringent and leaves him only a brief moment. How so? The Mishnah gives two examples:
"Nadrah b'leilei Shabbos" - a woman who took a vow on Friday night, and her husband heard about it, may annul it that very night, and also throughout the entire day of Shabbos until nightfall, nearly twenty-four hours.
"Nadrah im chasheichah" - if she took the vow close to sunset, at the very end of the day, he "meifer ela ad shelo techshach" - he may annul only until nightfall. "She'im chashechah v'lo heifer - eino yachol l'hafer" - for once night has fallen and he has not annulled, he can no longer annul, since the day has ended and it is no longer in his power to annul.
Annulling vows on Shabbos:
In passing, the Mishnah teaches that the annulment of vows may be done on Shabbos, and it makes no difference whether the vow or its annulment is for the needs of Shabbos or not. If she vowed not to eat on Shabbos, the annulment is certainly for the needs of Shabbos; but even when it is not for the needs of Shabbos, since the annulment of vows can only be done on the very day the husband heard, there is no other option: by the very definition of the law it must be done on Shabbos, for there is no other time available.
In this respect, the annulment of vows differs from the release of vows by a Beis Din, which is not done on Shabbos unless it is for the needs of Shabbos itself.